This invention relates to a test-signal generating device for testing electronic equipment and especially television equipment.
Test-signal generators are already known, specifically in the television field for controlling the operation of television transmitter-receivers. Generators of this type produce the majority of signals required for ordinary tests and a certain number of test signals are defined by international standards in order to facilitate the signal characterization operations or long-distance maintenance of television transmitter-receivers. Certain signals are inserted in the television image at lines which are well-determined by an international standard and are thus made invisible on the screens of television receivers. In this manner, testing and measuring operations remain entirely transparent for the user. Most of these generators are constructed in accordance with conventional analog techniques of signal formation.
In another known category are the digital or numerical signal generators, the function of which is to deliver a sequence of numerical values or samples describing the television test signal. This sequence is stored in a read-only memory (ROM) and is read at a suitable rate to be subsequently converted by a digital-to-analog converter which regenerates the desired television signal. The digital technique employed for delivering television test signals has a well-established reputation for providing reliable signals of good quality if the signals consist of a large number of samples per line and if each sample consists of a sufficient number of bits. A further advantage of the digital technique lies in the fact that it is also possible to obtain composite signals by selecting and combining different memories.
A problem arises, however, if it is desired to obtain a very wide variety of signals for digital test-signal generators. Inasmuch as a signal delivered by a conventional generator in the black and white mode, for example, requires between 1600 and 2048 eight-bit samples per scanning line in order to regenerate said signal, it is necessary to have a large capacity of read-only memories (ROMs) in order to obtain a large number of test signals. This capacity may amount to N.16 kilobits, where N represents the number of test signals stored in memory in the form of samples. Said capacity is appreciably increased in the case of color television signal generators in which the luminance signals require samples of at least ten bits and in which the chrominance samples (which are four times smaller in number than the luminance signals) require at least eight bits. In consequence, the capacity of the ROMs required in order to provide a sampled test-signal generator of the prior art increases and very rapidly exceeds reasonable limits when the number of test signals required attains only a few tens of signals.
Furthermore, the configuration of the test signals remains frozen in the ROM, with the result that it is impossible during a test to vary the signals or even to generate other signals which might seem to the operator to be better suited for the diagnosis obtained from the television equipment under test.